1. Field of the Invention
The present invention relates to a novel organic charge transfer complex which is excellent in both electrical conductivity and thermal stability, and to a process for preparation thereof. The present invention also pertains to a solid electrolytic capacitor employing said charge transfer complex.
2. Description of the Related Art
Electrically conductive organic charge transfer complexes are increasingly being put to practical use in many fields as, for example, materials capable of imparting electrical conductivity to general-purpose polymers, semiconductor devices, electrolytes for batteries, anode active materials for batteries, antistatic agents, heat sensitive materials, photoconductive materials, display devices, solid electrolytes for capacitors, etc. Conventional electrically conductive organic charge transfer complexes have useful properties characteristic to organic matter which are not found in metals, but are inferior in both electrical conductivity and thermal stability and it has therefore been desired that they be improved in respect of these properties.
Development of digital devices in recent years has given rise to a demand for large-capacity capacitors which have excellent high-frequency characteristics, i.e., relatively low impedance in the high-frequency region, and studies have been made to apply the above-described complexes to this field.
Examples of known capacitors which have excellent high-frequency characteristics include thin-film, mica and ceramic capacitors. These conventional capacitors, however, involve the problem that, when they are designed to obtain an electrostatic capacity of 1 .mu.F or more, their sizes are increased, and the production costs are also raised considerably.
Electrolytic capacitors, which are known to be large-capacity capacitors, include two types, that is, the liquid electrolyte type in which a liquid electrolyte is impregnated, and the solid electrolyte type in which manganese dioxide is employed as a solid electrolyte. The former type of electrolytic capacitor, which employs an electrolyte in a liquid state, utilizes ion conduction, and therefore resistance remarkably increases in the high-frequency region, so that the impedance of the capacitor increases disadvantageously. The latter type of electrolytic capacitor employs as a solid electrolyte manganese dioxide which is obtained by thermally decomposing manganese nitrate since manganese dioxide is an insoluble solid substance. This type of electrolytic capacitor also has relatively high impedance in the high-frequency region partly because manganese dioxide has a relatively high specific resistance, and partly because it is subjected to thermal decomposition to obtain manganese dioxide, and the oxide film thereof is readily damaged.
In order to overcome the above-described disadvantages of the conventional capacitors, one type of electrolytic capacitor which employs as a solid electrolyte a charge transfer complex consisting of a combination of 7,7,8,8-tetracyanoquinodimethane (hereinafter referred to as TCNQ) and a donor has already been proposed.
Examples of the donor employed in the proposed TCNQ charge transfer complex include N-n-hexylquinoline, N-ethylisoquinoline, N-n-butylisoquinoline (see Japanese Patent Public Disclosure No. 191414/1983), N-n-propylisoquinoline and N-iso-propylisoquinoline (see Japanese Patent Public Disclosure No. 17609/1983).
However, the conventional TCNQ charge transfer complex which may be composed of the above-described compounds suffers from inferior thermal stability and therefor involves a risk of the complex being decomposed during the capacitor manufacturing process such as to become an insulator. This prior art is also unsatisfactory in terms of electrical conductivity.